<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Title</title>
</head>
<body>
<div id="app">
    <button v-on:click="insertF">插入f</button>
    <ul>
        <li v-for="item in letters" :key="item">{{item}}</li>
    </ul>
</div>
<script src="https://unpkg.com/vue@3"></script>
<script>
    const app=Vue.createApp({
        data(){
            return{
              letters:['a','b','c','d','e']
            }
        },
        methods:{
            insertF(){
                this.letters.splice(3,0,'F')
            }
        }
    })
    app.mount("#app")
</script>
</body>
</html>